In the prior art, these kinds of pressure sensors comprising semiconductor chips were, as is disclosed in Japanese Patent Application Laid-Open No. H3-226638, equipped with a housing and a stem fixed to the lower portion of said housing and having a semiconductor chip storage chamber, wherein a glass seat is connected to the concave portion of said storage room, and a semiconductor chip is connected airtightly to said glass seat said semiconductor chip measuring the pressure of the refrigerant introduced through a pressure induction hole of said stem and a pressure induction hole of said glass seat.
In the above type of pressure sensors, a borosilicate glass is used as the material of the glass seat, and an iron-nickel (Fe--Ni) system alloy is used as the material of the stem, and these materials differ greatly in their temperature characteristics such as the thermal expansion coefficient. When temperature change occurs to a pressure sensor with such glass seat and metal stem connected to each other, the connection may be separated by the difference in the thermal expansion coefficient, which may lead to a problem where pressure could not be added accurately to the pressure sensing element.
In order to prevent such problems, the connection between the outer peripheral of the glass seat and the inner wall of the concave portion of the stem is provided by a low-melting glass having a thermal expansion coefficient which is at the middle of the thermal expansion coefficient of the glass seat and the thermal expansion coefficient of the stem, so that when the pressure sensor is used for a long period of time and repeated temperature changes occur to the connection of the glass seat and the stem, the connection will not separate or exfoliate, and the glass seat could be connected firmly to the upper portion of the stem.
The prior art pressure sensor of the above comprising the pressure sensing element formed of semiconductor chips had a problem in that the connection of the glass seat and the stem was troublesome and time-consuming, since the glass seat and the stem could not be connected by one step, having to place low-melting glass material to the gap between the outer peripheral of the glass seat and the concave of the stem, and then sealing the connection of the stem and the glass seat by melting said low-melting glass material.
Further, since the above-mentioned prior art technique connects the glass seat and the stem by a low-melting glass having a thermal expansion coefficient that is in the middle of the thermal expansion coefficient of each member, there is a need to select a glass material having a certain thermal expansion coefficient, which is inconvenient in that only a limited glass material could be used.
Further, the above technique solves the problem based on the difference in said expansion coefficient by selecting a certain glass material for the connection. However, the glass seat and the stem naturally differ in their thermal expansion coefficient based on the difference in material, so even when a low-melting glass having a thermal expansion coefficient in the middle of the thermal expansion coefficient of the glass seat and that of the stem is used, the connection provided by such technique may not be perfect when the pressure sensor is used for a long time.
In order to overcome the above problems, the present applicant proposed a pressure sensor including a connection of a stem and a seat for fitting a pressure sensing element of a semiconductor chip characterized in that the sensor is constituted so that the assembly and mounting of members could easily be done, and that the connected members do not separate or exfoliate under temperature change in Japanese patent application No. H8-268927.
The structure of the pressure sensor according to the above invention is explained hereinafter with reference to FIG. 3.
FIG. 3 is a vertical cross-sectional view of the pressure sensor 10 according to the above-explained invention. The pressure sensor 10 comprises a housing 11 of a metal material at the outer portion and an electric connector 12 which is fit and connected to said housing 11, and placed in the inner area formed by said housing 11 and said electric connector 12 is a semiconductor chip 13 with a pressure sensing element having a strain gage formed on the upper surface and an electric circuit board (print board) 14 with a calculating circuit and the like for converting an output from said semiconductor chip to an electric output of a voltage or a current.
Said housing 11 is formed of a metal such as iron (Fe) or stainless steel (SUS), and comprises a pipe-like screw 11c having a refrigerant liquid induction hole 11d on its lower portion and a receiving open pipe 11a on its upper portion, and further comprising a funnel-like supporting portion 11b in its middle interior portion, and said funnel-like supporting portion 11b includes a flat bottom 11e on its lower portion.
Said electric connector 12 is formed by an electrical insulating material of artificial resin such as polybutylene terephthalate reinforced by glass, comprising a guide pipe 12b for the fitting and separating of a connector on the upper portion, and an enlarging fitting pipe 12a on the lower portion. In the interior of said electric connector 12, three connector terminals 17 (two of which are not shown) penetrating the inner area of the enlarging fitting pipe 12a from the inner area of the guide pipe 12b are fixed.
Said electric circuit board 14 is placed to fit into an inner ring portion 11f of said receiving open pipe 11a of the housing 11, and an o-ring 15 is positioned on said electric circuit board 14 for waterproofing and supporting of the board 14, which is fit into said ring portion 11f. The fitting pipe 12a of said electric connector 12 fits to the upper end of said receiving open pipe 11a of the housing 11, and in a state where a lower inclining portion 12c of said fitting pipe 12a of the electric connector 12 presses said electric circuit board 14 through said o-ring 15, an opening peripheral portion 11a' at the end of said receiving open pipe 11a of the housing 11 stakes a shoulder portion 12d of said enlarging fitting pipe 12a of the electric connector. By such method, the housing 11 and the electric connector 12 are fit and connected together, and the electric circuit board 14 is fixed and held firmly inside the inner area formed by the housing and the electric connector.
A connecting conductive sleeve 18 is connected and fixed to a lower angled end 17a of said connector terminal 17 by a spot welding and the like, and on said connecting conductive sleeve 18, an upper portion 19b of a pin terminal 19 is inserted, and a lower portion 19a of said pin terminal 19 is fixed onto said electric circuit board 14 by soldering and the like. Said pin terminal 19 transmits electric signals from said electric circuit board to said connector terminal 17.
A stem 20 of an iron-nickel (Fe--Ni) alloy such as 42 alloy is fit and fixed to the funnel-like supporting portion 11b placed in the interior of said housing 11.
Said stem 20 comprises a lower pipe 20a which fits in the liquid induction hole 11d of said housing 11, a protruding collar 20b placed on a flat surface 11e of said funnel-like supporting portion 11b, and an upper mounting portion 20c for placing the seat 21, and further comprising a penetrating hole 20d in the upper and lower direction as a liquid path, wherein at least the surface contacting the seat 21 is gold (Au) plated with a thickness of approximately 1 .mu.m. Said stem 20 is welded firmly onto the plan surface 11e of said housing 11 by potential resistance weld at said protruding collar 20b.
A seat 21 made of silicon is positioned and fixed on said upper mounting portion 20c of the stem 20. Said seat 21 is equipped with a penetrating hole 21a for passing through liquid material which is positioned concentric to the penetrating hole 20d of said stem 20. An insulating layer of SiO.sub.2 with a thickness of approximately 1 .mu.m is formed on the upper surface of the seat 21 for electric insulation. Further, the surface of said insulating layer is gold (Au) plated with a thickness of approximately 1 .mu.m. Also, the lower surface of said seat 21 is gold (Au) plated so as to form a metal layer, and the seat is welded and fixed to the upper mounting portion 20c of said stem 20 by gold (Au) blazing.
A semiconductor chip 13 is positioned and fixed on said seat 21 for measuring the pressure of the refrigerant liquid. A diaphragm is formed on said semiconductor 13, and a semiconductor strain gage (both of which is not shown in the drawing) is positioned on said diaphragm. The pressure of the refrigerant introduced through the penetrating hole 20d of said stem and the penetrating hole 21a of said seat 21 is impressed to said diaphragm, and based on the level of pressure of said refrigerant, an electric signal is outputted from said semiconductor strain gage.
On the lower surface of said semiconductor chip 13 is also formed a gold (Au) plated metal layer with a thickness of approximately 1 .mu.m, wherein the semiconductor chip 13 is welded and fixed airtightly to said seat 21 by gold (Au) brazing.
On the upper portion of said electric circuit board 14 is a mount portion 14a of an electric circuit of an amplifying circuit and the like, and said mount portion 14a is electrically connected to the strain gage of said semiconductor chip 13 by a bonding wire 14b, and the electric signal from the strain gage of said semiconductor chip 13 is transmitted to the amplifying circuit of said electric circuit board 14 through said bonding wire 14b for amplification, which is outputted to said connector terminal 17 through said pin terminal 19.
Some other methods for connecting said housing 11 and said Fe--Ni alloy stem 20 are methods such as laser welding or silver or bronze brazing and the like. Further, in the case where the stem 20 is made of ceramic, an example of the method for connecting said housing 11 and said ceramic stem 20 is by mounting a connecting cap made of Fe--Ni alloy between the housing 11 and the ceramic stem 20, and the connection between each of said housing 11, said ceramic stem 20, said connecting cap and said seat 21 may be realized by forming a metal layer to the connecting portion of each said members by gold plating, and welding said metal layer by gold (Au) brazing.
However, even by said proposed methods, it is necessary to confirm the reliable connection between the protruding collar 20b of the stem 20 and the flat bottom 11e of the funnel-like supporting portion 11b of the housing 11, and to form an annular welding portion for completely shutting the area between the opening 11d and the space inside the funnel-like supporting portion 11b. However, it is difficult to form a connecting portion having a closed annular shape to such a wide contact surface by electrical resistance weld, and a problem such as stripping of the connecting portion may occur by using it for a long time.
The present invention considers the above problem, and aims at providing a pressure sensor comprising a pressure sensing element of a semiconductor chip formed by a plurality of resistors on a semiconductor board including a structure for fixing the pressure sensing element in the housing firmly and airtightly, and also at providing a structure for positioning the pressure sensing element and the calculation circuit in the same environment to improve measurement accuracy.